In the field of Radio Frequency (RF) communications, wireless communications devices in a wireless communications network, for example a cellular telecommunications network, possess transceiver circuits. It is known that transceiver circuits comprise, inter alia, a transmitter circuit.
Typically, a wireless communications network is designed and built so as to comply with one or more communications standards. One example of a communications standard is the Global System for Mobile communications (GSM) standard, which imposes strict requirements upon the function of a Mobile Subscriber (MS) handset that constitutes a wireless communications device. Of course, other Time Division Multiple Access (TDMA) communications standards exist as well as standards for other multiple access schemes.
In relation to the GSM standard (and others), an MS handset can transmit on a given channel. However, limitations are imposed upon power that can be generated in adjacent channels when transmitting on the given channel. In this respect, a so-called Switching Output Radio Frequency Spectrum (SxORFS) specification is associated with operation of the transmitter circuit, which operates in a burst mode.
Hence, it is important to maintain power control for the transmitter circuit throughout a transmission in order to comply with, inter alia, the SxORFS specification. In some operating conditions associated with the transmitter circuit, deviation from the SxORFS specification can occur, for example when a power amplifier of the transmitter circuit “ramps down” from a saturated state.
One known control loop circuit comprises a reference ramp generator arranged to generate a digital signal that is used to control a power amplifier that amplifies an input data signal to be transmitted by the transceiver circuit via an antenna. The digital signal has a profile that ramps up, maintains a level for a predetermined period of time and then ramps down again. The digital signal is fed to a digital-to-analogue converter and then low-pass filtered to yield a reference voltage signal. The reference voltage signal is fed to a summation unit that also receives a negative detection voltage signal. An output of the summation unit, constituting an error signal, is coupled to a controller that implements a proportional-integral control algorithm in order to yield an automatic power control voltage signal for controlling a bias of the power amplifier. A sample of an output signal of the power amplifier is obtained using a directional coupler, the sample of the output signal being processed by a detection unit capable of generating the detection voltage signal that is a measure of the power generated by the power amplifier, expressed as a voltage signal.
When operating, the power amplifier can run hot, a battery providing a supply voltage can be running low or the power amplifier can be transmitting in certain frequency “corners” that result in the power amplifier being unable to achieve a maximum output power of, for example, 33 dBm, when required to do so. This results in a persistent error signal due to a persistent difference between the reference voltage signal and the detection voltage signal. The controller therefore continually attempts to change the automatic power control voltage signal in order to try to achieve a maximum output power at the output of the power amplifier, and so the output power of the amplifier saturates and the bandwidth of the control loop collapses to zero; this state is known herein as “hard” saturation. Consequently, when the output power of the power amplifier ramps down at the end of a burst, the power amplifier must first “wind down” from the saturated state, and such winding down requires time to do so. However, in order to comply with the communications standard, the output power of the transmitter circuit has to ramp down within a predetermined period of time and a profile of the ramp down has to possess a predetermined shape, for example a raised-cosine function profile.
Of course, if a proportion of the time allotted for ramp-down is used to wind the power amplifier down, the power amplifier has to complete the ramp-down according to the raised-cosine profile in the remaining (less) time and so the ramp-down has a steeper gradient than would otherwise be the case if it was not necessary to unwind from the saturated state. Consequently, the likelihood of out-of-band interference being generated, i.e. in adjacent channels, is increased.
One known partial solution to the generation of the out-of-band interference is disclosed in US patent publication no. US 2004/0176049 A1, where first and second error signals are generated and used when applying a limit reference input (AOC_MAX) of a controller of a radio communications transmitter. However, the circuit proposed in US 2004/0176049 A1 does not cure generation of the out-of-band interference in some circumstances.
In some instances, the circuit can achieve a target output power so that a detected error between actual output power and the target output power is zero. When this occurs, the power amplifier can be sufficiently compressed such that satisfactory SxORFS performance for a ramp down event cannot be guaranteed. In this respect, upon reaching the target output power, the bandwidth of a control loop comprising the power amplifier collapses to a non-zero value below a minimum threshold value and so the loop is unable to maintain lock; this state is known herein as “soft” saturation. Alternatively, another saturation state can occur when the output power reaches the target output power, but the bandwidth of the control loop collapses to zero; this state is known herein as “virtual” saturation. Hence, it can be seen that bandwidth and thus lock of the control loop are not always guaranteed in relation to the circuit of US 2004/0176049.